Optical temperature measurement technique utilizing phosphors

ABSTRACT

A technique wherein an object or environment to be measured is provided with a phosphor material layer that emits at least two optically isolatable wavelength ranges whose intensity ratio depends upon the object or environment temperature. This technique is applied to remote hostile environment point temperature measurements, such as in large enclosed electrical power transformers and other large equipment, to the measurement of surface temperatures, such as in airplane structures being tested in wind tunnels, and elsewhere.

BACKGROUND OF THE INVENTION

This invention relates generally to devices and methods for makingtemperature measurements, and more specifically to devices and methodsthat make such measurements by optical techniques that utilizetemperature-sensitive phosphors.

There are many methods currently used for temperature measurement. Themost common industrial techniques utilize thermocouples, thermistors orresistance thermometers by means of which electrical signals aregenerated and then converted into temperature readings or employed forcontrol functions.

On occasion, however, it is useful, and sometimes essential, to obtaintemperature data by non-electrical techniques. This may occur: (1) wheretemperatures over large areas are to be measured and measurement by adense distribution of thermocouples thus becomes impractical; (2) wherethe attachment of thermocouples and leads would alter the temperaturesto be measured; (3) in environments where, because of high electric ormagnetic fields, metallic wires are undesirable; (4) where electricalisolation and/or insensitivity to electrical noise generation isdesired; (5) where, because of motion or remoteness of the part to besensed, permanent lead wires are impractical; or (6) where, because ofcorrosive chemical environments, wires and thermocouple junctions wouldbe adversely affected, with resultant changes in electricalcharacteristics. In these situations, optical techniques frequentlybecome preferable.

The most direct optical technique for temperature measurement isinfrared radiometry. However, where line of sight measurement is notpossible, without infrared transmission media, the infrared techniquessuffer a disadvantage. In such an instance there are relatively fewmaterials sufficiently transparent to long-wave infrared radiation toprovide an infrared conducting path from the area where temperature isto be sensed to the infrared detector. Furthermore, infrared techniquesare not absolute in that the emissivity of the emitting material has tobe known accurately if the infrared radiometric measurements are to beconverted into true temperature readings.

Optical pyrometers can also be used, but only for very hot sources whichemit visible radiation. Optical pyrometers also suffer from the sameproblems as infrared radiometers when it comes to absolute measurements.

For large area measurements, thermographic phosphors or liquid crystalsare sometimes employed in the form of films, paint or coatings appliedto the surface to be measured. Known typical thermographic phosphorsexhibit a broad fluorescence under ultraviolet excitation, thisfluorescence being strongly temperature-dependent with regard toemission intensity. The fluorescent intensity of this emission"quenches" sharply as the temperature rises over a fairly narrowtemperature range. It is difficult to calibrate a thermographic phospherabsolutely because changes in excitation, such as might be caused bysource instability, can be misinterpreted as a temperature variation.Liquid crystals change their reflected colors with temperature over asimilarly narrow range. Both materials suffer from the fact that, toachieve high sensitivity, the range over which the materials willoperate as temperature sensors is of necessity fairly restrictedcompared with the materials of this invention. Most liquid crystalmaterials are also relatively unstable and may change their chemical andphysical properties over a period of time. While this is not always aproblem, it can be in selected applications.

Therefore, it is a primary object of the present invention to providetechniques for remote temperature measurement using optical rather thanelectrical techniques that permit elimination of metallic wires,junctions and connectors, that circumvent electrical noise sources andthat provide for measurement over extended areas as well as pointmeasurements.

It is another object of the present invention to provide an internallycalibrated phosphor temperature measuring system whereby changes intotal fluorescent intensity with time as might be caused by a variationin excitation, changes in optical transmission with time or changes insensitivity of a receiving detector with time are not interpreted astemperature changes.

It is yet another object of the present invention to provide a means ofmeasuring temperatures of objects or environments without the necessityof direct physical contact with electrical wires, such as situationswhere the point to be measured is submerged in a corrosive gas orliquid, must be isolated electrically or thermally, is in a vacuum, oris located on a moving part to which permanent leads cannot beconveniently connected.

Finally, it is an object of the present invention to provide a means ofmaking absolute, internally calibrated temperature measurements overwider temperature ranges than would be possible with conventionalthermographic phosphors or liquid crystals.

SUMMARY OF THE INVENTION

These and additional objects are accomplished by the techniques of thepresent invention wherein, generally, an object or environment for whicha temperature is to be measured is provided with a layer of phosphormaterial that when excited to luminescence emits detectable radiationwithin two or more distinct wavelength ranges that are opticallyisolatable from one another, with a relative intensity of emission inthese wavelength ranges varying in a known manner as a function of thetemperature of the phosphor. Such a phosphor material may be a singlephosphor composition exhibiting such characteristics or may,alternatively, be two or more phosphor compositions in a physicalmixture that together exhibit these characteristics. Sharp line emittingphosphors, such as those having rare earth activators, are preferred. Apractical system of accurately measuring temperatures over wide rangesis thus made possible, a normal desired range of from -100° C to +400° Cbeing achievable.

The intensity of two such lines of phosphor emission are detected and aratio of the detected signals taken. The ratio is convertible intotemperature is accordance with the known temperature characteristics ofthe phosphor material. This optical system is internally calibratedbecause the taking of a ratio makes the technique relatively insensitiveto changes in total intensity of the phosphor emissions, general changesin optical transmission or changes in the sensitivity of the receivingdetector which may occur in time. The technique is thus adapted for longterm remote temperature measurement applications.

The use of this approach permits several specific temperaturemeasurement improvements and solves heretofore unsolved temperaturemeasuring problems. According to one more specific form of theinvention, remote, non-contact temperature measurements can be made oflarge surface areas, such as those in models being tested in windtunnels, by painting the phosphor over the surface areas to bemonitored. The model is then illuminated by an appropriate excitingradiation and intensity measurements of the selected phosphorluminescent lines are taken of selected points on the model from outsideof the wind tunnel. Heating of the model surface by a flow of airthereover is thus monitored.

According to another specific aspect of the invention, remotemeasurement of point temperatures are made possible. Temperatures deepinside an apparatus, for instance, are extremely difficult to measure,and heretofore have not been measured in environments where metallicwires cannot be used. One such environment is in large electrical powertransformers that are sealed, filled with oil, operated at hightemperatures and have high levels of electric and magnetic fields thatwill not tolerate insertion of any metallic parts of a more conventionaltemperature measurement system. According to the present invention, thephosphor material is formed internal to a small sensor on the end of along fiber optic cable. The sensor is then immersed in the location ofthe transformer where a spot temperature measurement is needed. Thephosphor is coupled to the detector by means of the fiber optic cablewith the measurements of the phosphor luminescence being made outside ofthe apparatus.

The present invention has been described only very generally. Additionalobjects, advantages and features thereof are set forth as part of thefollowing description of the preferred embodiments of the variousaspects of the present invention, which should be taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating in general the basic aspects ofthe present invention;

FIG. 2 are curves that illustrates the fluorescent emission spectrum attwo different temperatures of europium-doped lanthanum oxysulfidephosphor when excited by ultraviolet radiation;

FIG. 3 are curves that illustrate the intensity of specific strongemission lines from a europium-doped lanthanum oxysulfide phosphor whenexcited by ultraviolet radiation;

FIG. 4 schematically illustrates one specific form of the presentinvention wherein the temperature of the surface of a wind tunnel modelis remotely measured;

FIG. 5 shows one specific form of an optical detector 103 of thetemperature measuring system of FIG. 4;

FIG. 6 shows another specific form of an optical detector 103 of thetemperature measurement system of FIG. 4;

FIG. 7 schematically illustrates a large electrical power transformerutilizing one aspect of the present invention for remotely measuringspot temperatures thereof;

FIG. 8 shows a phosphor temperature sensor and optical system thereforas one form of the temperature measurement system of FIG. 7;

FIG. 9 shows a variation in the temperature measurement system of FIG.8;

FIG. 10 shows yet another variation of the temperature measurementsystem of FIG. 8;

FIG. 11 illustrates a rotating device with its internal temperaturebeing measured according to another aspect of the present invention;

FIG. 12 illustrates a moving belt with its temperature being measuredaccording to another aspect of the present invention, and

FIG. 13 illustrates another aspect of the present invention wherein thetemperature of fluid flow is measured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the basic features of all of the various aspects ofthe present invention are illustrated. Within some environment 1 ispositioned a solid object 20 having a phosphor coating 40 over at leasta portion thereof. The phosphor is characterized by emitting, whenexcited, electromagnetic radiation within separable bandwidths at two ormore distinct wavelengths and with relative intensities in those bandsthat vary as a known function of the temperature of the phosphor 40.Thus, the temperature of the phosphor 40 is detected that is the same asor related to that of the object 20, and in some applications of theenvironment 1 as well.

Such luminescent emission of the phosphor 40 in the form ofelectromagnetic radiation 41, generally in or near the visible spectrum,is excited by a source 60 over a path 61. The source could beradioactive material, a source of cathode rays, an ultravioletelectromagnetic energy source, or any other remote source producingefficient fluorescence depending upon the particular type of phosphorutilized in the preferred forms of the present invention. The relativeintensities of two distinct wavelength bands within the emittedradiation 41 contains the desired temperature information.

The emitted radiation 41 is gathered by an optical system 80 anddirected in a form 81 onto an optical filter and radiation detectorblock 100. The block 100 contains filters to isolate each of the twobands or lines of interest within the radiation 81 that contain thetemperature information. After isolation, the intensity of each of thesebands or lines is detected which results in two separate electricalsignals in lines 101 and 102, one signal proportional to the intensityof the radiation in one of the two bands and the other signalproportional to the intensity of the radiation in the other of the twobands of interest.

These electrical signals are then applied to an electronic signalprocessing circuit 120. In a preferred form, the signal processingcircuits 120 take a ratio of the signals in the lines 101 and 102 by theuse of routinely available circuitry. This electronic ratio signal isthen applied to a signal processor within the block 120. The signalprocessor is an analog or digital device which contains the relationshipof the ratio of the two line intensities as a function of temperaturefor the particular phosphor 40 utilized. This function is obtained bycalibration data for the particular phosphor 40. The output of thesignal processor in a line 121 is thence representative of thetemperature of the phosphor 40.

The signal in the line 121 is applied to a read out device 140 whichdisplays the temperature of the phosphor 40. The device 140 could be anyone of a number of known read out devices, such as a digital or analogdisplay of the temperature over some defined range. The device 140 couldeven be as elaborate as a color encoded television picture wherein eachcolor represents a narrow temperature range on the object. It could alsobe a television picture stored on disc or tape.

PREFERRED PHOPSHOR MATERIALS AND CHARACTERISTICS

The fundamental characteristics of a phosphor material for use in thepresent invention is that when properly excited it emits radiation in atleast two different wavelength ranges that are optically isolatable fromone another, and further that the intensity variations of the radiationwithin each of these at least two wavelength ranges as a function of thephosphor temperature are known and different from one another. Aphosphor material is preferred that is further characterized by itsradiation emission in each of these at least two wavelength bands beingsharp lines that rise from substantially zero emission on either side toa maximum line intensity, all in less than 100 angstroms. The lines areeasy to isolate and have their own defined bandwidth.

For a practical temperature measuring device, the phosphor materialselected should also emit radiation in the visible or near visibleregion of the spectrum since this is the easiest radiation to detectwith available detectors, and since radiation in this region is readilytransmitted by glass or quartz windows, fibers, lenses, etc. It is alsodesirable that the phosphor material selected be an efficient emitter ofsuch radiation in response to some useful and practical form ofexcitation of the phosphor material. The particular phosphor material ormixture of phosphor materials is also desirably chosen so that therelative change of intensity of emission of radiation within the twowavelength ranges is a maximum within the temperature range to bemeasured. The phosphor material should also be durable, stable and becapable of reproducing essentially the same results from batch to batch.In the case of fiber optic transmission of the phosphor emission, asdescribed in specific embodiments hereinafter, a sharp line emittingphosphor is desirably selected with the lines having wavelengths nearone another so that any wavelength dependent attenuation of the fiberoptic will not significantly affect the measured results at a positionremote from the phosphor, thereby eliminating or reducing the necessityfor intensity compensation that might be necessary if fibers of varyinglengths were used.

The composition of a phosphor material capable of providing thecharacteristics outlined above may be represented very generally by thegeneric chemical compound description A_(x) B_(y) C_(z), wherein Arepresents one or more cations, B represents one or more anions,together forming an appropriate non-metallic host compound, and Crepresents one or more activator elements that are compatable with thehost material. x and y are small integers and z is typically in therange of a few hundredths or less. Since the activator element isprincipally responsible for the emission characteristics of the phosphorwith respect to wavelength bands and temperature dependence of emissiontherein, it is the selection of the activator element(s) of a phosphorcompound that will principally determine whether the phosphor isappropriate for use in carrying out the present invention. The choice ofthe host compound will also have secondary effects on the detailedspectrun emitted, excitation efficiency, material characteristics suchas transmission and durability, etc. There are a large number of knownexisting phosphor compounds from which those satisfying the fundamentalcharacteristics discussed above may be selected by a trial and errorprocess. A preferred group of elements from which the activator elementC is chosen is any of the rare earth elements having an unfilledf-electron shell, all of which have sharp isolatable radiation lineemissions of 10 angstroms bandwidth or less. Certain of these rare earthelements having comparatively strong visible or near visible emissionare preferred for convenience of detecting, and they are typically inthe trivalent form: praseodymium (Pr), samarium (Sm), europium (Eu),terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er) and thulium(Tm). Other non-rare earth activators having a characteristic of sharpline emission which might be potentially useful in the present inventionwould include uranium (U) and chromium (Cr³⁺). The activator element iscombined with a compatable host material with a concentration ofsomething less than 10 atom percent relative to the other cationspresent, and more usually less than 1 atom percent, depending on theparticular activator elements and host compounds chosen.

A specific class of compositions which might be included in the phosphorlayer 40 is a rare earth phosphor having the composition (RE)₂ O₂ S:X,wherein RE is one element selected from the group consisting oflanthanum (La), gadolinium (Gd) and yttrium (Y), and X is one dopingelement selected from the group of rare earth elements listed in thepreceeding paragraph having a concentration in the range of 0.01 to 10.0atom percent as a substitute for the RE element. A more usual portion ofthat concentration range will be less than 1.0 atom percent and in somecases less than 0.1 atom percent. The concentration is selected for theparticular emission characteristics desired for a given application. Aswill be noted below, europium (Eu³⁺) could be used alone in the abovenoted hosts to provide the desired optical and thermal characteristicswhereas the other activators mentioned above would in all likelihood beused singly as activators and then physical mixtures of two or more suchresultant phosphor compounds would be prepared to produce a phosphormaterial having the desired optical and thermal characteristics.

Such a phosphor compound may be suspended in a nitrocellulose binder forapplication in environments having temperatures of less than about 120°F, and in a silicate binder for higher temperature applications.

A specific example of such a material for the phosphor layer 40 of FIG.1 that is very good for many applications is europium-doped lanthanumoxysulfide (La₂ O₂ S:Eu) where europium is present in the range of oneatom percent down to 0.1 atom percent as a substitute for lanthanum. Thecurves 42 and 43 of FIG. 2 provide, for two separate phosphortemperatures, the intensity of its emission as a function of wavelength.The phosphor was in the form of a finely crystalline powder and wasexcited by electrons. The emitted radiation was analyzed with a scanningmonochromator followed by a photo-multiplier detector. The particularmaterial for which FIG. 2 illustrates the fluorescent emission spectrumis lanthanum oxysulfide with 0.1 atom percent of europium substitutedfor lanthanum.

Curve 42 of FIG. 2 shows the emission spectra of such a material at 295°K which is room temperature. The curve 43 of FIG. 2 shows the emissionspectra for the material at 77° K, the extremely cold temperature ofliquid nitrogen. It will be noted that the spectral characteristics ofthe emission are much different at these two temperatures and thesechanges continue to occur as the phosphor is raised above roomtemperature.

Narrow wavelength fluorescent lines which are particularly useful fortemperature measurement, as marked on the curves of FIG. 2, are locatedat approximately 4680 angstroms, 5379 angstroms, 5861 angstroms(actually a doublet) and 6157 angstroms. The relative intensities ofthese lines change as a function of temperature of the phosphor and itis these relative intensities that give the temperature information inthe various forms of the present invention.

The relative intensities of at least two of suitable narrow bandwidthspectral lines are determined, in the preferred forms of the invention,by taking the ratio of the detected intensities of two of the lines. Thetwo lines must thus be non-overlapping and separated enough inwavelength so that their intensities may be measured independently.Referring to FIG. 3, the intensities of the four spectral linesidentified on FIG. 2 are drawn as a function of temperature of thephosphor. Additionally, curve 44 of FIG. 3 shows a ratio of the twospectral lines at 5379 angstroms and 4680 angstroms as a function oftemperature. It is such a characteristic as illustrated by the curve 44that permits accurate, pre-calibrated temperature measurement by takinga ratio of two spectral lines. Similarly, if the other two lines at 6157angstroms and 5861 angstroms are ratioed, the characteristic of theresulting ratio as a function of temperature is given in curve 45. Ascan be seen from FIG. 3, the ratio represented by the curve 44 variesstrongly within a temperature range from -75° C to +50° C. The secondratio indicated by the curve 45, on the other hand, varies strongly withtemperature over the range of from about 50° C to 300° C. Therefore, theparticular fluorescent emission spectral lines of the phosphor that areutilized depend upon the expected temperature range to be monitored.

In order to adequately detect and measure these spectral line ratioswithout interference from adjacent emission lines, the fluorescentradiation 41 and 81 of FIG. 1 must first be passed, as part of the block100, through an optical filter such as a monochromator or interferencefilter set chosen to isolate the selected wavelength ranges in which thespectral lines of interest fall. It can be seen from the characteristicsof the phosphor illustrated in FIG. 2 that for the 4680 angstrom, 5861angstrom and 6157 angstrom lines, a bandpass filter in the order of 50angstroms wide is adequate for separation. In addition to separation, itmay also be desirable to correct the measured lines intensities withinthe block 100 for any strong background radiation which may be present,such as that from room light or day light. For that purpose, it may bedesirable in certain circumstances to additionally measure the intensityof radiation as seen through the utilized monochromator or filter whentuned to a spectral region near the fluorescent lines but where nofluorescent radiation is expected. An example using the phosphor whosecharacteristics are illustrated in FIG. 2 is in the region of from 6000to 6100 angstroms. Alernatively, the background can be determined byturning off the excitation source and looking through the two filters.Any background radiation so measured can then be subtracted from the5861 and 6157 angstrom line intensities that are measured to yield amore correct ratio for temperature measurement purposes.

A physical mixture of phosphor compounds can also be utilized, as analternative, in order to obtain desired temperature characteristics. Theintensity of one emission line from one compound of the mixture, forinstance, can be compared with the line intensity of another compound inorder to provide optimum measuring characteristics over a giventemperature range. Alternatively, two emission lines from each of twophosphor compositions can be utilized, the lines from one compoundcompared over one temperature range and the lines from the othercompound being compared over an adjacent temperature range. For example,a terbium doped lanthanum, gadolinium or yttrium oxysulfide may be usedas one compound in combination with an europium-doped lanthanum,gadolinium or yttrium oxysulfide as the other compound.

The phosphor materials mentioned above have an advantage of beingrelatively inert and stable. The emission lines of the phosphor are inthe visible or near visible region and thus transmission through longair paths, through water and other liquids, or through long opticalfibers, or through glass or quartz optics, is possible. Such a phosphordiffers from more conventional phosphors in that it emits very sharpline output spectra that can be readily optically isolated from eachother, and the temperature dependence of line intensities at aparticular wavelength is very strong relative to that at otherwavelengths over a given temperature range of practical interest. Otherphosphor materials having these characteristics can be utilized as partof the technique and structure of the various aspects of the presentinvention, as well.

REMOTE NON-CONTACT SURFACE TEMPERATURE MEASUREMENTS

Referring to FIG. 4, an object 21 within an environment 2 has itsoutside surface painted with phosphor material 46. By monitoring theemission of the phosphor, when properly excited, the surface temperatureof the object 21 can be monitored from a remote distance and withoutcontacting the object 21.

In the particular example shown in FIG. 4, the object 21 is anaerodynamic model positioned in an environment 2 that is a test windtunnel. The surface temperature being monitored on the model 21 providesinformation as to the effect of the air flow in heating the modelsurface.

The phosphor painted on the surface of the model 21 is excited toluminescence by illumination from ultraviolet lamps 62 and 63. In somesituations, an ultraviolet laser might be used as well, particularly formeasurement of selected object points. The ultraviolet output of thelamps 62 and 63 are passed, respectively, through windows 64 and 65 thatare transparent to ultraviolet energy so that it might pass into thewind tunnel 2 and onto the model 21. Another window 82 permits emittedradiation from the phosphor on the surface of the model 21 to begathered by an optical system, represented by lenses 83 and 84. Thecollected radiation 85 is then directed onto a filter and detectorsystem 103. The filter and detector 103 is similar to the filter anddetector 100 previously described with respect to FIG. 1.

Referring to FIG. 5, details of one form of the filter and detector 103are illustrated. A filter wheel 104 is positioned in the path of theradiation 85 from the phosphor. The wheel 104 has at least two differentfilters 105 and 106 spaced on different areas of the wheel 104 so thatas it is rotated by the motor 112 the filters 105 and 106 arealternatively passed through the beam 85. The filters 105 and 106 aredesigned to be narrow bandpass filters to select out two differentspectral lines of the phosphor being utilized.

The two selected phosphor emission lines are thus applied in timesequence to a detector 107 whose output is applied to an electroniccircuit 108. The detector could be a photomultiplier or a siliconphoto-diode which would give only an average of the intensity of theparticular selected lines over the entire object 21 or the detector 107could be some other device, such as an image dissector or a televisioncamera, that would convert the optical image of the object 21 as viewedby the selected emission lines into a two dimensional intensity plot.The use of the latter type detectors has an advantage of permittingtemperature detection on each point of the object 21 separately. Theelectronics 108 receives a synchronous signal from the detector 111which tells it which of the two filters 105 and 106 are in front of thedetector 107 at any instant. This permits the electronics 108 to developthe two signals 109 and 110 representative, respectively, of theintensities of the two selected emission lines of the phosphor.

FIG. 6 shows another form of the filter and detector 103 of FIG. 4. Inthe form of FIG. 6, a beam splitter or dichroic mirror 90 is positionedin the path of the phosphor fluorescent emission beam 85 so that knownfractions of the intensity of the beam goes in each of two directions.One direction is through a filter 115 and onto a single detector 116 todevelop an electrical signal 110'. The other path is through a filter113 onto a second detector 114 to develop a signal 109'. Each of thefilters 113 and 115 are selected to permit one or the other of twoselected emission spectral lines to pass therethrough and onto theirrespective detectors. The output signals in the lines 109 and 110 ofFIGS. 4 and 5, and 109' and 110' of FIG. 6, are applied to appropriatesignal processing and readout circuits as described with respect toblocks 120 and 140 of FIG. 1. The read-out device would depend, ofcourse, upon the type of detector used, being a television displaysystem or video storage medium if the detector 107 is a televisioncamera.

REMOTE POINT TEMPERATURE MEASUREMENT

There are many applications of large machinery and apparatus wherein itis desired to monitor the temperature at one or more points within theapparatus while it is operating. Large machinery is especiallyexpensive. It is very inconvenient and expensive when it breaks down dueto local overheating. If such local overheating can be detected beforeany damage is done, then the cause of it can be determined, thusavoiding more costly shutdowns of the equipment. Monitoring the overallor average temperature of the equipment, by monitoring the temperatureof water or oil coolant, for instance, does not provide the necessaryinformation in most instances because the overheating could be raisingthe temperature of a small part of the machinery to an excessive anddamaging level without raising the average temperature any detectableamount.

One such piece of equipment wherein there has been a long need for suchpoint temperature measurement is in large electrical power transformers,some of which are capable of handling several megawatts of electricalpower. Destruction of such a large piece of equipment is not onlyextremely costly but can significantly disrupt a large portion of anelectric power company's distribution system. The problem has not beensatisfactorily solved before since electrical transformers, as is thecase with other high voltage electrical equipment, cannot tolerate anyelectrical conductors within the equipment that will disturb theelectric and magnetic fields or cause a potential for short circuits.Therefore, there is a need for a non-metallic local point temperaturesensor that can be used inside of electrical power transformers or othertypes of large electrical equipment.

Referring to FIG. 7, such a transformer is very generally illustrated. Athick steel outer shell 7 contains a transformer core 6 having windings4 and 5 therearound. The entire core and windings are submersed in anoil bath 3 for insulation and cooling. In order to monitor thetemperature of a given spot on the interior of such a transformer, asingle sensor 22 is provided in accordance with another aspect of thepresent invention. The sensor 22 is connected to one end of a longoptical fiber bundle 86. The sensor 22 may be constructed without anymetal parts at all and is optically connected by the fiber bundle 86 toan appropriate filter and detector system 100', an electric signalprocessing circuit 120' and a direct temperature read-out device 140'.

Referring to FIG. 8, the temperature sensor 22 is shown in cross sectionwherein it contains a phosphor material 87 in optical communication withone end of the optical fiber bundle 86. This end of the optical fibersand the phosphor are all sealed together by an appropriate glass orceramic material to form a probe which may be inserted into atransformer or other machinery. The probe is subjected to thetemperature to be measured and the phosphor, being part of that probe,responds as described hereinbefore with relative changes in theintensity of its spectral output lines as a function of temperature.

The output of the phosphor 47 is obtained at an opposite end of thefiber bundle 86 by a lens 87 which directs the emission radiationthrough a beam splitter or dichroic mirror 88, through another lens 89,and thence to a system already described with respect to FIG. 6,including a beam splitter or dichroic mirror 90, two filters 113 and 115and two radiation detectors 114 and 116.

In order to excite the phosphor 47 to emit the desired lines, theembodiment of FIG. 8 employs an ultraviolet light source 66 whose outputis directed by a lens 67, passed through a broadband ultraviolet filter68 which blocks all but the ultraviolet light and then onto the beamsplitter or dichroic mirror 88. The element 88 is designed to transmitvisible light but reflect ultraviolet light so that the opticalconfiguration shown in FIG. 8 utilizes such a characteristic toadvantage. The ultraviolet radiation is reflected by the element 88,directed through the lens 87 into the optical fiber bundle 86 andtransmitted through it to the phosphor material 47 to excite itsluminescent emission which provides the temperature information in acoded form, as described above.

FIG. 9 shows a variation of the probe and detecting system of FIG. 8wherein a probe 23 includes a phosphor material 48 attached to one endof an optical fiber bundle 91. Encapsulated within the probe 23 in thisembodiment is a radioactive material 69 which is selected to excite, fora number of years, the phosphor material 48. The emission of thephosphor material 48 is transmitted through the optical fiber bundle 91,through a lens 92 and onto a beam splitter, filter and detector systemas described previously with respect to FIGS. 6 and 8. The radioactivematerial 69, used in place of the ultraviolet source 66 of FIG. 8, maybe, for example, an isotope of nickel, such as ₆₃ Ni, having a half lifeof 92 years. This material emits electrons but does not emit gamma rays.This probe 23 and communicating optical fiber bundle 91 still maymaintain the desirable characteristic of having no metallic component ifthe ₆₃ Ni is in the form of an oxide or other non-metallic compound.

FIG. 10 shows a variation of either of the probe assemblies of FIGS. 8and 9 wherein a single optical fiber bundle 92 provides opticalcommunication with a plurality of separate probes, such as the probes24, 25 and 26, which can be positioned at different locations within apower transformer or other apparatus. At one end of the optical fiberbundle 92, a few of the fibers are connected with each of the individualprobes 24, 25 and 26. At the opposite end of the fiber bundle 92, theopposite ends of the same optical fibers are connected to individualfilters and detectors. That is, the probe 24 is in optical communicationwith only the filter and detector block 117, the probe 25 only with thefilter and detector block 118, and so forth. Alternatively, the separateprobes can be scanned at the output end of the fiber optic bundle by asingle detector in a controlled and predetermined fashion.

Obviously, the specific types of equipment where such temperature probeshave a high degree of utility are numerous. An electric power generatingnuclear reactor is another place where the invention can be used withgreat advantage to measure temperature of remote, inaccessiblepositions.

OTHER APPLICATIONS

The techniques of the present invention lend themselves to opticalcommutation. They may be applied without physical contact and are immuneto electrical noise. A specific applicaton of optical commutation is ona rotating device 200 as shown in FIG. 11. This device could be a motor,turbine or generator. The phosphor containing probe 22 is embedded inthe rotating part 200 as are an optical fiber input bundle 210 and anoutput bundle 203. The optical fiber bundles terminate at an externalcircumference of the wheel or rotating part 200. This permits thenon-rotatable fixed positioning of an exciting radiation source, such asan ultraviolet source 205, and phosphor emission receiving optics 207adjacent thereto. At one position, for a short instance, in eachrotation of the rotating part 200, the ultraviolet source and thephosphor emission radiation optics 207 will be aligned with theirrespective optical fiber bundles 201 and 203. At that instant, thetemperature of the part at the position of the embedded phosphorcontaining probe 22 is measured. The optical system 207 is connectedwith an appropriate filter and detector 209 of one of the typesdiscussed with respect to other of the embodiments above.

The same technique can be utilized, as shown in FIG. 12, for a movingbelt 211. This optical temperature measurement technique can be seen tohave considerable advantages since no physical connection of wires orother devices are required between the moving part and the fixedmeasuring equipment. As an alternative to the particular opticaltechnique shown in FIGS. 11 and 12, the rotating part 200 and the belt211 could also be painted with a phosphor paint as discussed withrespect to FIG. 4.

Referring to FIG. 13, yet another application of the basic concept ofthe present invention is shown wherein the temperature of a movingstream of fluid 215 passing through a pipe 217. A window 219 is providedin the wall of the pipe 217 and characterized by transmittingultraviolet and visible radiation without significant attenuation. Anelectromagnetic energy source 221 in the ultraviolet spectrumilluminates the interior of the pipe through the window 219. The fluidstream 215 is provided with a plurality of phosphor coated particles 223that have a size and density consistent with the type of fluid 215 andflow to be expected so that they remain distributed within the fluidstream 215. The radiation from the ultraviolet source 221 causes thephosphor coating on the particles 223 to luminesce and this luminescenceis gathered by an optical system 225 which collects and transmits thephosphor radiation to an appropriate detector 227. By detecting andratioing the intensities of two phosphor emission lines of interest, thetemperature of the fluid stream 215 is determined since the particleshave been given a chance to reach a temperature equilibrium with that ofthe fluid stream 215.

Other particular applications will become apparent from thisdescription. The probe and optical fiber embodiment can be applied topoint temperature measurement in humans and animals, for example.

The technique can also be broadly applied to imaging thermographywherein an object scene is imaged onto a phosphor screen and theemissions detected through filters by a television camera to measure therelative intensity of two emission lines and thence the temperature ofthe image, the latter being proportioned to the temperature of theobject scene. In yet another approach to thermal imaging, the phosphorscreen could be mounted within a vacuum tube, illuminated from one sideby the thermal image, via a suitable infrared-transmitting window andsubstrate, and excited from the other side by an electron beam scannedin raster fashion. In this instance the thermal image could bereconstructed using a single pair of optical point detectors suitablyfiltered with the resultant line intensity ratio thence used to modulatethe intensity of the electron beam of a cathode ray display tube whichis also scanned in raster fashion in synchronism with the excitingelectron beam.

Although the various aspects of the present invention have beendescribed with respect to a preferred embodiment thereof, it will beunderstood that the invention is entitled to protection within the fullscope of the appended claims.

I claim:
 1. A method of determining the temperature of an object,comprising the steps of:positioning a layer of phosphor material in aheat conductive relationship with said object, said phosphor beingcharacterized by emitting, when excited, electromagnetic radiationwithin optically isolatable bandwidths at at least two distinctwavelength ranges and with relative intensities therein that vary as aknown function of the phosphor temperature, exciting said phosphor tocause emission of said at least two wavelength bands, opticallydirecting said phosphor emission to a detecting position, and detectingthe relative intensities of said at least two distinct wavelength bands,whereby said relative intensity is an indication of the temperature ofthe phosphor layer and related thereto by said known temperatureemission function.
 2. A method as defined by claim 1 wherein the step ofpositioning a layer of phosphor material comprises the step of coatingsaid phosphor material directly on the surface of said object.
 3. Amethod as defined by claim 1 wherein the step of optically directingsaid phosphor emission to a detecting station comprises imaging theexcited phosphor onto said detecting station, and further wherein thestep of detecting the relative intensity of said at least two distinctwavelength radiation bands comprises detecting selected points of saidimage.
 4. A method as defined by claim 1 wherein the step of opticallydirecting said phosphor emission to a detecting station comprisesimaging the excited phosphor onto said detecting station, and furtherwherein the step of detecting the relative intensity of said at leasttwo distinct wavelength radiation bands comprises the step of averagingsubstantially all of said image intensity by directing it onto a singledetector.
 5. A method as defined by claim 4 wherein the step ofpositioning a layer of phosphor material additionally comprises the stepof positioning therein a phosphor composition (RE)₂ O₂ S:X wherein RE isan element selected from the group consisting of lanthanum, gadoliniumand yttrium, and wherein X is a doping element with a concentration offrom 0.01 to 1.0 atom percent and is selected from the group consistingof europium, terbium, praseodymium, samarium, dysprosium, holmium,erbium and thulium.
 6. A method as defined by claim 1 wherein saidobject comprises a probe designed for immersion into an environmentwhose temperature is to be measured, and further wherein the step ofoptically directing said phosphor emission to a detecting stationincludes positioning between said phosphor and said detecting station alength of optical fiber.
 7. A method as defined by claim 6 wherein thestep of exciting said phosphor comprises the step of positioningadjacent to said layer of phosphor material a source of electrons orother radioactive emissions.
 8. A method as defined by claim 1 whereinthe step of exciting said phosphor comprises the step of directingelectromagnetic energy against said phosphor.
 9. A method as defined byclaim 1 wherein the step of detecting the relative intensities of saidat least two distinct wavelength radiation bands comprises the steps ofdetecting separately each of the relative intensities of said at leasttwo distinct wavelength radiation bands, and taking the ratio of thedetected intensities.
 10. A method as defined by claim 1 wherein thestep of positioning a layer of phosphor material comprises the step ofpositioning therein a phosphor composition additionally characterized byits said at least two wavelength ranges being sharp lines of emissioneach rising from substantially zero emission to a peak in less than 100angstroms bandwidth.
 11. A method of remotely measuring the temperatureof an environment, comprising:positioning an object in said environmentin a manner that the object is heated by the environment, positioning alayer of phosphor material in heat conductive relationship with saidobject, said phosphor being characterized by emitting, when excited,electromagnetic radiation within optically isolatable bandwidths at atleast two distinct wavelength ranges and with relative intensitiestherein that vary as a known function of the phosphor temperature,exciting said phosphor to cause emission of said at least two wavelengthbands, positioning a fiber optic between said phosphor to a remotemeasuring position in a manner to optically transfer the emittedradiation from the phosphor to the remote measuring position, anddetecting the relative intensities of said at least two distinctwavelength radiation bands at the remote end of the fiber optic bundle.12. A method as defined by claim 11 wherein the step of exciting thephosphor includes positioning a source of electrons or other radioactiveemissions adjacent to or as part of said object.
 13. A method as definedby claim 11 wherein the step of exciting the phosphor includes the stepof passing electromagnetic radiation in the ultraviolet range from saidremote location through an optical fiber to said phosphor.
 14. A methodas defined by claim 11 wherein the step of positioning a phosphormaterial comprises the step of positioning therein a phosphorcomposition additionally characterized by its said at least twowavelength ranges being sharp lines of emission each rising fromsubstantially zero emission to a peak in less than 100 angstromsbandwidth.
 15. A method as defined by claim 11 wherein a plurality ofsuch objects are positioned with their associated phosphor layers invarious different locations within said environment, another fiber optictransferring the emitted radiation from each object location to theremote position, the radiation from each fiber optic being detectedseparately in space or time at the remote measuring position.
 16. Amethod as defined by claim 11 wherein said environment is a piece oflarge equipment such as a sealed power transformer with said objectpositioned therewithin at a point where the temperature is to bemonitored and said remote measuring position is outside saidtransformer.
 17. A device for remotely measuring temperature,comprising:a length of a fiber optics, a layer of phosphor material heldat one end of said fiber optics in light communication therewith, saidphosphor being characterized by emitting, when excited, electromagneticradiation within optically isolatable bandwidths at at least twodistinct wavelength ranges and with relative intensities therein thatvary as a known function of the phosphor temperature, means for excitingsaid phosphor to cause emission of said at least two wavelength bands,means at said one end of the fiber optics sealing the phosphor layertherewith, whereby said one end constitutes a probe that may be insertedinto an environment whose temperature is to be measured, means at anopposite end of said fiber optical bundle to filter from the phosphoremission transmitted therethrough each of said at least two distinctwavelength bands, means for independently detecting the intensity ofeach of said at least two wavelength bands as received at the oppositeend of the fiber optic bundle, and means for comprising the detectedintensities, whereby the temperature of said probe can be remotelydetermined.
 18. The device of claim 17 wherein said comparison meansincludes means for taking a ratio of the detected phosphor wavelengthband intensities.
 19. The device according to claim 17 wherein saidexciting means includes a source of electrons or other radioactiveemissions permanently sealed within said probe in a position immediatelyadjacent said phosphor layer.
 20. The device according to claim 17wherein said exciting means comprises at said opposite end of the fiberoptics:means for generating electromagnetic energy within theultraviolet or visible range, and optical means for directing saidultraviolet or visible radiation through said fiber optics to itsphosphor end in a manner that does not interfere with said detectingmeans.
 21. The device according to claim 17 wherein said phosphor isadditionally characterized by its said at least two wavelength rangesbeing sharp lines of emission each rising from substantially zeroemission to a peak in less than 100 angstroms bandwidth.
 22. A systemfor determining the temperature of an object, comprising:a layer ofphosphor material applied to said object in intimate contact therewith,said phosphor being characterized by emitting, when excited,electromagnetic radiation within optically isolatable bandwidths at atleast two distinct wavelength ranges and with relative intensitiestherein that vary as a known function of the phosphor temperature, meansfor exciting said phosphor material to cause emission of said at leasttwo wavelength bands, means for independently detecting the intensity ofradiation emitted by the phosphor material and each of said at least twowavelength bands, and means for comparing the detected intensities,whereby the temperature of said object can be remotely determined. 23.The system according to claim 22 wherein said phosphor is additionallycharacterized by its said at least two wavelength ranges being sharplines of emission each rising from substantially zero emission on eitherside to a peak line intensity in less than 100 angstroms bandwidth.